In many applications, an intelligence bearing signal is encoded, transmitted, and received in the form of plural blocks of data. Depending on the manner in which the blocks were generated, and depending upon their ultimate use, it is frequently necessary to transpose the blocks of data from the order in which they are received to a different order conforming with their intended use.
For example, in a television system which processes digital video pictures, each digital video picture is typically divided into a rectangular array having plural lines of video data, where each line of video data is comprised of a plurality of pixel values. The video data for what is known as an interlaced video sequence of pictures is characterized by a frame rate which is typically 30 Hz, and each frame is composed of two interlaced fields. The first field consists of the odd numbered lines of the frame (i.e., the lines numbered 1, 3, 5, . . . ), and the second field consists of the even numbered lines of the frame (i.e., the lines numbered 2, 4, 6, . . . ).
This interlaced video sequence may be encoded, typically by an MPEG-2 video encoder. The MPEG-2 video encoder may arbitrarily choose to encode the video data of a picture as either a frame or as separate fields. If the MPEG-2 video encoder chooses frame encoding, the video picture data is effectively encoded and transmitted in frame order (i.e., lines 1, 2, 3, 4, . . . are transmitted, in order, for that picture). If the MPEG-2 video encoder chooses field encoding, the video picture data is effectively encoded and transmitted in field order (i.e., lines 1, 3, 5, . . . followed by lines 2, 4, 6, . . . are transmitted, in order, for that picture). The MPEG-2 video encoder may change its encoding decision from picture to picture.
An MPEG-2 video decoder which receives this encoded video picture data must decode this interlaced video sequence and produce decoded pixel values in field order so that the pixel values can be displayed by a video display device which displays video data as interlaced fields. This decoding must be done whether the incoming encoded video picture data is in field order or frame order. Thus, when the incoming encoded video picture data is in frame order, the frame ordered decoded data must be converted to field order before it is sent to the interlaced display device. As discussed immediately below, the conversion of frame ordered video data to field ordered video data typically requires some amount of memory because lines of pixels must be output by a frame to field convertor in a different order than the order in which they were received and decoded.
That is, for the case of I and P type pictures, MPEG video decoding requires that these types of pictures be decoded and stored in memory so that the pixel data comprising these pictures may be used in the processing of other pictures. These I and P type pictures do not exit the decoder until they have been completely decoded and stored in memory. However, in the case of B type pictures, MPEG video decoding does not require storage because B type picture data is not used in the processing of other pictures. B type pictures can in principle be decoded and sent to the display device without storing them in memory.
When I and P type pictures are decoded in frame order and are stored in memory as required, they can easily be read out of memory in field order. No additional memory needs to be added to the system in order to achieve this conversion. B type pictures encoded in frame order, however, require some amount of memory to be added to the system so that they can be converted to field order.
A simple way of performing this conversion is to use two additional frame-size memories. As frame or field order data for picture n is stored in the first of these memories, the data for picture n-1 previously stored in the second of these memories is read out in field order. Then, when frame or field order data for picture n+1 is received, this data is stored in the second of these memories as the data for picture n previously stored in the first of these memories is read out in field order. This type of convertor requires a memory size of two frames, which adds to the expense of the converter.
The present invention is directed to a frame to field converter which employs a memory having a reduced size.